Method to detect a defective element

ABSTRACT

The present invention relates to a method to detect at least one defective pixel in a spatial light modulator comprising numerous pixel elements. The spatial light modulator is imaged to a detector. A relayed image of a first chess-board pattern of pixels in said spatial light modulator is detected by said detector. A relayed image of a second chess-board pattern of pixels in said spatial light modulator is detected, which is inverted to the first chessboard pattern, by said detector. The relayed images of said first and second chessboard patterns are analyzed to detect differences between said detected images and theoretical images thereof.

RELATED APPLICATIONS

The present application claims the benefit of and priority to U.S.Provisional Application Ser. No. 60/440,145, filed on 15 Jan. 2003,entitled Method to Detect a Defective Element.

TECHNICAL FIELD

The present invention relates to a detection method, in particular itrelates to a method to detect partially or completely defective pixelsin a spatial light modulator.

BACKGROUND OF THE INVENTION

Modern UV-lithography is searching for new highly parallel writingconcepts. Spatial light modulation (SLM) with optical MEMS devicesoffers such possibilities. Special emphasis must be put on the abilityof SLM devices to handle ultraviolet light (UV), deep ultraviolet light(DUV) and extreme ultraviolet light (EUV).

A SLM chip may comprise a DRAM-like CMOS circuitry with several millionindividually addressable pixels on top. Said pixels are deflected due toan electrostatic force between a mirror element and an addresselectrode.

For good performance of the SLM chip almost every single pixel must workand moreover they must respond to an applied voltage similarly enough tobe able to compensate for the differences by calibration.

Using a white light interferometer has earlier created a map ofdefective pixels. Software identified the individual pixels andcalculated the deflection angles automatically. Such a method findsquite well not only fabrication errors, but also additional errors thatoccurred during operation. On the other hand such method is rather slow,as a high magnification implies that many images have to be taken. Asthe size of the SLM chips are growing, and thereby the number of pixels,more and more pixels has to be analyzed. Preferably the SLM chips alsohave to be analyzed in a faster way than what is done today. Therefore,what is needed in the art is a faster way to get information aboutdefective pixels of a particular SLM chip.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod for detecting defective pixels in a spatial light modulator withreduced process time.

This object, among others, is according to a first aspect of theinvention attained by a method to detect one or a plurality of defectivepixels in a spatial light modulator, comprising the actions of:providing an electromagnetic radiation source to illuminate said spatiallight modulator, arranging a reference pattern in said spatial lightmodulator, illuminating said spatial light modulator, determining aposition of a reference pixel in said spatial light modulator bydetecting a relayed image of said reference pattern in a detectorarrangement, arranging a first pattern in said spatial light modulator,illuminating said spatial light modulator, detecting a relayed image ofsaid first pattern in said detector arrangement, arranging at least asecond pattern in said spatial light modulator, illuminating saidspatial light modulator, detecting a relayed image of said at least asecond pattern in said spatial light modulator, analyzing said relayedimages of said first pattern and said at least a second pattern todetect differences between said images and theoretical images thereof.

In another embodiment according to the present invention said first andsecond patterns are chessboard patterns, where the first chessboardpattern is inverted to the second chessboard pattern.

In still another embodiment according to the invention the relayed imageis detected by a CCD camera.

In yet another embodiment according to the present invention theprojection of a SLM pixel is bigger than a CCD pixel.

In yet another embodiment according to the present invention singlepixels in the spatial light modulator are not resolved in said detector.

In yet another embodiment according to the present invention a spatialfilter between the detector and the spatial light modulator is adaptedto vary the degree of resolution on said detector.

In yet another embodiment according to the present invention at leastone of said first and second patterns is detected by illuminating saidpattern at least twice and detecting the relayed images separately.

In yet another embodiment according to the present invention at leastone of said first and second patterns is comprised of only non deflectedand fully deflected pixels.

In yet another embodiment according to the present invention said fullydeflected pixels correspond to a maximum degree of extinction by meansof diffraction.

In yet another embodiment according to the present invention saidchessboard patterns are comprised of only non-deflected and fullydeflected pixels.

In yet another embodiment according to the present invention saidchessboard patterns are comprised of only non-deflected and partiallydeflected pixels.

In yet another embodiment according to the present invention said fullydeflected pixels corresponds to a maximum degree of extinction by meansof diffraction.

In yet another embodiment according to the present invention saidpartially deflected pixels corresponds to partial extinction by means ofdiffraction.

In yet another embodiment according to the present invention said firstand second patterns are each detected a plurality of times, where thepixels in said patterns are set to different degrees of deflectionbefore each detection event.

In yet another embodiment according to the present invention saidchessboard patterns are comprised of only fully-deflected and partiallydeflected pixels.

In yet another embodiment according to the present invention saidchessboard patterns are comprised of pixels being in a first partiallydeflected state and a second partially deflected state.

The invention also relates to a method to detect at least one defectivepixel in a spatial light modulator comprising numerous pixel elements,comprising the actions of: detecting a relayed image of a firstchessboard pattern of pixels in said spatial light modulator by saiddetector, detecting a relayed image of a second chessboard pattern ofpixels in said spatial light modulator, which is inverted to the firstchessboard pattern, by said detector, analyzing the relayed images ofsaid first and second chessboard patterns to detect differences betweensaid detected images and theoretical images thereof.

In another embodiment according to the present invention a CCD cameradetects the relayed images.

In still another embodiment according to the present invention a SLMpixel is bigger than a CCD pixel.

In yet another embodiment according to the present invention singlepixels in the spatial light modulator are not resolved in said detector.

In yet another embodiment according to the present invention a spatialfilter between the detector and the spatial light modulator is adaptedto vary the degree of resolution of said relayed image on said detector.

In yet another embodiment according to the present invention at leastone of said first and second patterns is detected by illuminating saidpattern at least twice and detecting the relayed images separately.

In yet another embodiment according to the present invention saidchessboard patterns are comprised of only non-deflected and fullydeflected pixels.

In yet another embodiment according to the present invention saidchessboard patterns are comprised of only non-deflected and partiallydeflected pixels.

In yet another embodiment according to the present invention said fullydeflected pixels correspond to a maximum degree of extinction by meansof diffraction.

In yet another embodiment according to the present invention saidpartially deflected pixels correspond to partial extinction by means ofdiffraction.

In yet another embodiment according to the present invention said firstand second patterns are each detected a plurality of times, where thepixels in said patterns are set to different degrees of deflectionbefore each detection event.

In yet another embodiment according to the present invention saidchessboard patterns are comprised of only fully-deflected and partiallydeflected pixels.

In yet another embodiment according to the present invention saidchessboard patterns are comprised of pixels being in a first partiallydeflected state and a second partially deflected state.

In yet another embodiment the present invention further comprising theaction of identifying a SLM reference pixel in a detector pixel grid.

The invention also relates to a method to detect at least one defectivepixel in a spatial light modulator, comprising the action of making animage of a first chessboard pattern unsharp so that a regular chessboardpattern becomes a uniform background at a detector plane and a defectivepixel becomes an irregularity in said uniform background at said planeand detectable by a detector.

In another embodiment the present invention further comprising theaction of making an image of a second chessboard pattern, which secondpattern is inverted to said first pattern, unsharp so that a regularchessboard pattern becomes a uniform background at a detector plane anda defective pixel becomes an irregularity in said uniform background atsaid plane and detectable by a detector.

In yet another embodiment of the present said detector is a CCDcamera.34.

In yet another embodiment according to the present invention aprojection of a SLM pixel onto said CCD is bigger than a CCD pixel.

In yet another embodiment according to the present invention said firstor second chessboard patterns are comprised of only non-deflected andfully deflected pixels.

In yet another embodiment of the present invention said first or secondchessboard patterns are comprised of only non-deflected and partiallydeflected pixels.

In yet another embodiment according to the present invention said fullydeflected pixels correspond to a maximum degree of extinction by meansof diffraction.

In yet another embodiment according to the present invention saidpartially deflected pixels correspond to partial extinction by means ofdiffraction.

In yet another embodiment according to the present invention said firstor second chessboard patterns are comprised of only fully-deflected andpartially deflected pixels.

In yet another embodiment said first or second chessboard patterns arecomprised of pixels being in a first partially deflected state and asecond partially deflected state.

In yet another embodiment said invention further comprising the actionof identifying an SLM reference pixel in a detector pixel grid.

Further characteristics of the invention, and advantages thereof, willbe evident from the detailed description of preferred embodiments of thepresent invention given hereinafter and the accompanying FIGS. 1–10,which are given by way of illustration only, and thus are not limitativeof the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a test unit for detecting defective SLM pixels.

FIG. 2 depicts a flow diagram over an embodiment of a test procedureaccording to the invention.

FIG. 3 depicts a perspective view of a camera pixel and a SLM pixel,which are aligned to each other center to center.

FIG. 4 depicts a perspective view of a camera pixel and a SLM pixel,which are not aligned with each other.

FIG. 5 depicts the illumination from the chip pixel in FIG. 3 onto saidcamera pixels.

FIG. 6 depicts the illumination from the chip pixel in FIG. 4 onto saidcamera pixels.

FIG. 7 depicts the output of the camera for the illumination in FIG. 5.

FIG. 8 depicts the output of the camera for the illumination in FIG. 6.

FIG. 9 depicts a checkerboard pattern.

FIG. 10 depicts a defective off pixel in a checkerboard pattern.

FIG. 11 depicts a defective on pixel in a checkerboard pattern.

FIG. 12 depicts aerial cross sectional images of defects.

DETAILED DESCRIPTION

The following detailed description is made with reference to theFigures. Preferred embodiments are described to illustrate the presentinvention, not to limit its scope, which is defined by the claims. Thoseof ordinary skill in the art will recognize a variety of equivalentvariations on the description that follows.

Further, the preferred embodiments are described with reference to ananalogue spatial light modulator (SLM). It will be obvious to oneordinary skill in the art that there may be situations when other SLMsthan analogue ones will be equally applicable; for example digital SLMslike a digital micromirror device DMD made by Texas instruments.Additionally, SLMs may comprise reflective or transmissive pixels.

The invention relates to a method to detect defects in the SLM. Such amethod is useful when patterning a workpiece using said spatial lightmodulator (SLM).

FIG. 1 illustrates an optical test unit for detecting defective pixelsin the spatial light modulator. The optical test unit is built similarto a lithographic pattern generator. The optical test unit comprises alaser source 110, a beam homogenizing and shaping device 120, a mirror130, a beam splitter 150, a spatial light modulator 140, a Fourier lens160, a spatial filter 170, an imaging lens 180 and a detecting device190 for monitoring the aerial image.

The laser source 110 may be an excimer laser emitting 248 nm DUV pulses.Said pulses is homogenized and shaped by the homogenizing and shapingdevice 120. Said device 120 comprises optics such that plane wavesexpose the surface of the SLM 140. As deflection angles of SLM pixels inan analogue spatial light modulator is very small, in the range of microradians if the shape of the SLM pixel is rectangular with sides being 16micro meters long, the surface of it has to be considered as a blazedgrating. In any area with deflected pixels the light is diffracted intonon-zero orders and absorbed by an aperture or spatial filter 170. Onlyvery little stray light reaches the detecting device 190 in thecorresponding area. The detecting device 190 may be a CCD camera. TheFourier lens 160 ensures that the performance is the same for each pixelposition on the SLM. For flat areas with non-deflected pixels most ofthe light is reflected as a zeroth diffraction order, transferred by thespatial filter 170 and focused by the imaging lens 180 to create abright area in the CCD camera 190. This is illustrated by a solid rayconstruction starting at point 142 in FIG. 1. Starting point 142 may bea single or a plurality of pixel elements in the spatial lightmodulator.

A deflected SLM pixel, diffract a non-negligible amount of light inhigher diffraction orders, such as the first, second third etc. Thespatial filter 170 blocks diffraction orders higher than a predetermineddiffraction order, being for example the first diffraction order. Thisis illustrated by a dashed ray construction starting at point 144 inFIG. 1. The point 144 may represent a single or plurality of pixels in adeflected state. The deflection of the pixel will extinguish a certainamount of the light reflected into the zeroth order because ofdestructive interference. The more the pixel is deflected the more thelight will be extinguished (for small deflections only, after maximumextinction some intensity will come back), and at a certain degree ofdeflection maximum extinction is achieved. By increasing the size of theaperture 170 the image would be sharper and a checkerboard pattern wouldnot give a uniform gray background. The reason for this is that higherorders of diffraction would pass said aperture. An increase in theaperture 170 also affect the degree of deflection for reaching the samedegree of extinction as with a smaller aperture 170, a bigger aperturerequires the pixel element to deflect more for achieving the same levelof extinction given the same size of the pixel element.

An optical demagnification may be chosen such that one SLM-pixel isimaged to approximately 2 by 2 camera pixels. Due to the spatial filterthe optics does not fully resolve single pixels. An embodiment, whichdoes not fully resolve single square shaped SLM pixels with sides being16 μm long, has a numerical aperture NA of 0.009 at an image plane, herethe detecting device 190, and a numerical aperture NA of 0.0045 at anobject plane, here the spatial light modulator 140, with an illuminatingwavelength of 248 nm. With another demagnification, pixel size andwavelength said NA would be different. Therefore, the image of forexample a “black” pixel in a “white” background is imaged as an unsharpdark spot. Appearance and peak value in the camera depend on therelative position of the SLM pixel to a camera pixel grid, see FIGS.3–8. The sum of pixel values, however, is almost independent of thisrelative alignment. A position of a defective pixel is found bycalculating the center of gravity of a plurality of camera pixel values.

FIG. 3 illustrates a camera pixel 310, aligned with a single defectiveSLM pixel 320. As the projected size of the SLM pixel is most probablynot an integer multiple of the camera pixel size, some of the SLM pixelshappen to sit symmetrically on (and around) a camera pixel, while thecenter of other SLM pixels fall on the border between camera pixels.Most of the SLM pixels will have an arbitrary position without symmetrywith respect to a camera pixel grid. As long as the camera pixels aresmaller than the SLM pixels this will not affect the performance of theinventive detection method.

FIG. 5 depicts the illumination of the defective SLM pixel 320 onto thecamera pixels 310. As can be seen from the Figure, the size and shape ofthe illumination is different to a real size of the SLM pixel. This is aresult of diffraction and properties of the optics not to resolve singlepixels. The result is a blurred image of the SLM pixel, which affect agreater number of camera pixels than would have been the case withoptics resolving said single pixel. The center of gravity of saidillumination may easily be determined by comparing detected illuminationof different camera pixels.

FIG. 4 illustrates an example of when the center of a defective SLMpixel 420 falls on the border between two camera pixels 410, 412.

FIG. 6 depicts the illumination of the defective SLM pixel 420 onto thecamera pixel grid. As can be seen from FIG. 6, the size and shape of theillumination is equal to the one depicted in FIG. 5, the only differenceis the position of the center of gravity of said illumination in saidcamera pixel grid.

FIG. 7 illustrates an output from the detecting device 190 when beingilluminated as depicted in FIG. 5. FIG. 8 illustrates the output fromthe detecting device 190 when being illuminated as depicted in FIG. 6.

By a coordinate transformation an absolute position of a defective SLMpixel can be calculated. To find parameters for this coordinatetransformation an image with a few straight lines may be written to theSLM chip. These may be recognized in the CCD camera response by a Houghtransformation, which is very insensitive to noise and defects.

Commercial CCD cameras may be too small to view the whole SLM chip atone time. The CCD camera may comprise 1536 by 1024 pixels, which mayonly be enough to cover a portion of SLM pixels, for instance about onethird of the number of SLM pixels. Therefore, only a portion of said SLMmay be tested at one time. The SLM can be mounted on a slider andshifted to different positions and the test may be repeated for eachposition.

FIG. 2 illustrates a flow chart of an embodiment of a test procedureaccording to the present invention. Defective pixels may be divided intodifferent categories. Pixels may be weakly or not responding to anaddress signal. This might be a mechanical or an electrical problem. AnSLM pixel may always be deflected. The reason for this may be that thepixels are stuck to an electrode, or permanent damage to hinges attachedto said pixels. The pixel may reflect electromagnetic radiation poorly.The cause for that may be dust particles on the pixel, a fabricationdefect or laser damage to the mirror. Lastly, an electrical short cutbetween adjacent pixels may cause said pixels to always deflect in thesame manner.

Defects of the above mentioned categories might be found by an inventiveprocedure, which in its simplest version only comprises three images ofeach position of the SLM. One image is used for position reference andthe other two for detecting defective pixels. The positioning of the SLMrelative the CCD camera may be performed as follows. By using thereproductive principle of pattern writing on the chip, white lines maybe regularly written on a black background. White lines on a blackbackground are superior to black lines in a white background, due to thefact that the background around the chip is dark. No pre-process is thennecessary to eliminate the chip surrounding. Lines may be recognized ina robust way bay means of a Hough transformation, allowing independencefrom noise. With line equations, parameters such as intersection points,scale and angle may be computed.

Due to the finite resolution of the imaging optics with the spatialfilter 170, a chessboard pattern free of defects is transferred to auniform gray background in the camera. Single defective pixels show upas dark or bright spots, respectively.

A single not responding pixel in a chessboard pattern will result in abright spot that should be easily distinguishable from the background.Possible not responding pixels that are supposed to be flat in thischessboard pattern will always be found in the inverted chessboardpattern. Pixels that are erroneously deflected or poorly reflecting willsimilarly create a dark spot.

The test procedure as illustrated in FIG. 2 starts with a chip positioninitialization 210, which means that a particular area of the SLM chipis going to be investigated. Thereafter a characteristic pattern iswritten on the SLM for position recognition 220. Position recognitioncomprises an alignment of the SLM chip with the CCD camera. When thealignment is finished a defect detection of the SLM chip can start. Apicture of a chessboard pattern is taken in step 230; said picture isanalyzed in step 240. Step 250 checks if every kind of chessboardpattern has been written to the SLM chip. If yes, a comparison if thesingle analysis results are performed 260. If no, next pattern iswritten to the SLM chip 255. Step 270 checks if every position of theSLM is investigated. If no, next position is analyzed 275. If yes adefective pixel map is built out of the results.

A defect free SLM with a chess-board pattern, which is detected with aCCD camera at a detector plane, will appear in the camera as a uniformgray picture. The detector plane is at equal or equivalent distance tosaid spatial light modulator as an image plane in a pattern generatorusing such a spatial light modulator as a programmable reticle/mask.Defects will appear as stains, darker or brighter than the surroundingdepending on the type of defect. In case of a noise, which is notuniform, stains may be hidden in said noise. In case it is possible toknow the nature of noise (source, statistical parameter ordeterministic), a preprocess can be used to reduce or eliminate theeffect of said noise. Time averaging can reduce statistical noise.Filters for statistical or time varying deterministic noise depend onthe type of noise. If the noise is deterministic and time unvarying, areference picture featuring the noise can be subtracted to the noisypicture in question. The ideal is to have a reference mirror that fillsthe whole image.

Comparing a theoretical image of a relayed pattern in the SLM and adetected pattern performs the analyzing step. Deviations in the detectedpattern from the theoretical one correspond to defective pixel(s).

FIG. 9 illustrates a typical chessboard or checker board pattern 900. Achessboard pattern combines many advantages when trying to detectdefective pixels. If one pixel does not have the value it is supposed tohave, a spot of five pixels (the defective pixel and its neighbors) willdisturb enough of the intensity to appear in the CCD camera. Anotheradvantage of the chessboard pattern is that its opposite pattern is achessboard as well, which means that only two pictures are needed toobserve the behavior of all the pixels at a given voltage. With theinventive method it is possible to find all defective pixels with only asmall number of measurements. In one embodiment this is achieved bymaking an image of the chessboard pattern, and its inverted pattern,just so unsharp that a regular chessboard pattern becomes a uniformbackground in a detector plane, but that both erroneously deflected orflat pixels will stick out distinctively of this uniform background.

FIG. 10 illustrates a defective pixel 1010 in a chessboard pattern. Thedefective pixel appears as a black pixel, i.e., fully deflected whichmay be caused by an electrical short cut between an adjacent deflectedpixel and this one, which is supposed to be non deflected.

FIG. 11 illustrates a defective pixel 1110 in another chessboardpattern, which chessboard pattern is the inverted pattern to thechessboard pattern in FIG. 10. The defective white pixel, which issupposed to be black, may be stiff and cannot be deflected. A mirror ofpoor reflectivity (a grey pixel surrounded by 4 black pixels), will bedetected as a gray spot in the CCD camera.

FIGS. 9–11 illustrate pixels in the chessboard pattern being black andwhite pixels. However, a chessboard pattern may be constructed withpixels being white and grey, black and grey or light grey and dark grey.

FIG. 12 illustrates a cross section of a resulting aerial image ofdefective pixels in a gray background. A horizontal line 1220 representsa background intensity that would be the response of an undisturbedcheckerboard pattern. A pixel added, i.e., a white pixel, which issupposed to be black (see FIG. 11), is represented by curve 1230 and apixel lacking, i.e., a black pixel, which is supposed to be white (seeFIG. 10) is represented by curve 1210. The asymmetry in FIG. 12 comesfrom the fact that the regular checkerboard pattern gives only 25% lightintensity in the zeroth diffraction order, which is 50% of lightamplitude.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is understood that theseexamples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

1. A method to detect one or a plurality of defective pixels in aspatial light modulator, comprising the actions of: providing anelectromagnetic radiation source to illuminate said spatial lightmodulator, arranging a reference pattern in said spatial lightmodulator, illuminating said spatial light modulator, determining aposition of a reference pixel in said spatial light modulator bydetecting a relayed image of said reference pattern in a detectorarrangement, arranging a first pattern in said spatial light modulator,wherein features in an area of the first pattern are too small andclosely spaced to be individually resolved by the detector arrangement,illuminating said spatial light modulator, detecting a relayed image ofsaid first pattern in said detector arrangement, arranging at least asecond pattern in said spatial light modulator, illuminating saidspatial light modulator, detecting a relayed image of said at least asecond pattern in said spatial light modulator analyzing said relayedimages of said first pattern and said at least a second pattern todetect differences between said images and theoretical images thereof.2. The method according to claim 1, wherein said first and secondpatterns are chessboard patterns, where the first chessboard pattern isinverted to the second chessboard pattern.
 3. The method according toclaim 1, wherein the relayed image is detected by a CCD camera.
 4. Themethod according to claim 3, wherein the projection of a SLM pixel isbigger than a CCD pixel.
 5. The method according to claim 1, whereinsingle pixels in the spatial light modulator are not resolved in saiddetector.
 6. The method according to claim 5, wherein a spatial filterbetween the detector and the spatial light modulator is adapted to varythe degree of resolution on said detector.
 7. The method according toclaim 1, wherein at least one of said first and second patterns isdetected by illuminating said pattern at least twice and detecting therelayed images separately.
 8. The method according to claim 1, whereinat least one of said first and second patterns is comprised of only nondeflected and fully deflected pixels.
 9. The method according to claim8, wherein said fully deflected pixels corresponds to a maximum degreeof extinction by means of diffraction.
 10. The method according to claim2, wherein said chessboard patterns are comprised of only non-deflectedand fully deflected pixels.
 11. The method according to claim 2, whereinsaid chessboard patterns are comprised of only non-deflected andpartially deflected pixels.
 12. The method according to claim 10,wherein said fully deflected pixels corresponds to a maximum degree ofextinction by means of diffraction.
 13. The method according to claim11, wherein said partially deflected pixels corresponds to partialextinction by means of diffraction.
 14. The method according to claim 2,wherein said first and second patterns are each detected a plurality oftimes, where the pixels in said patterns are set to different degrees ofdeflection before each detection event.
 15. The method according toclaim 2, wherein said chessboard patterns are comprised of onlyfully-deflected and partially deflected pixels.
 16. The method accordingto claim 2, wherein said chessboard patterns are comprised of pixelsbeing in a first partially deflected state and a second partiallydeflected state.
 17. A method to detect at least one defective pixel ina spatial light modulator comprising numerous pixel elements, comprisingthe actions of: detecting a relayed image of a first chessboard patternof pixels in said spatial light modulator by said detector, whereinsquares of the first chessboard pattern are too small and closely spacedto be individually resolved by said detector, detecting a relayed imageof a second chessboard pattern of pixels in said spatial lightmodulator, which is inverted to the first chessboard pattern, by saiddetector, analyzing the relayed images of said first and secondchessboard patterns to detect differences between said detected imagesand theoretical images thereof.
 18. The method according to claim 17,wherein the relayed images are detected by a CCD camera.
 19. The methodaccording to claim 18, wherein the projection of a SLM pixel is biggerthan a CCD pixel.
 20. The method according to claim 17, wherein singlepixels in the spatial light modulator are not resolved in said detector.21. The method according to claim 20, wherein a spatial filter betweenthe detector and the spatial light modulator is adapted to vary thedegree of resolution of said relayed image on said detector.
 22. Themethod according to claim 17, wherein at least one of said first andsecond patterns is detected by illuminating said pattern at least twiceand detecting the relayed images separately.
 23. The method according toclaim 17, wherein said chessboard patterns are comprised of onlynon-deflected and fully deflected pixels.
 24. The method according toclaim 17, wherein said chessboard patterns are comprised of onlynon-deflected and partially deflected pixels.
 25. The method accordingto claim 23, wherein said fully deflected pixels corresponds to amaximum degree of extinction by means of diffraction.
 26. The methodaccording to claim 24, wherein said partially deflected pixelscorresponds to partial extinction by means of diffraction.
 27. Themethod according to claim 17, wherein said first and second patterns areeach detected a plurality of times, where the pixels in said patternsare set to different degrees of deflection before each detection event.28. The method according to claim 17, wherein said chessboard patternsare comprised of only fully-deflected and partially deflected pixels.29. The method according to claim 17, wherein said chessboard patternsare comprised of pixels being in a first partially deflected state and asecond partially deflected state.
 30. The method according to claim 17,further comprising the action of: identifying a SLM reference pixel in adetector pixel grid.
 31. A method to detect at least one defective pixelin a spatial light modulator, comprising the action of: making an imageof a first chessboard pattern unsharp so that a regular chessboardpattern becomes a uniform background at a detector plane and a defectivepixel becomes an irregularity in said uniform background at said planeand detectable by a detector.
 32. The method according to claim 31,further comprising the action of: making an image of a second chessboardpattern, which second pattern is inverted to said first pattern, unsharpso that a regular chessboard pattern becomes a uniform background at adetector plane and a defective pixel becomes an irregularity in saiduniform background at said plane and detectable by a detector.
 33. Themethod according to claim 31, wherein said detector is a CCD camera. 34.The method according to claim 33, wherein a projection of a SLM pixelonto said CCD is bigger than a CCD pixel.
 35. The method according toclaim 31, wherein said chessboard pattern is comprised of onlynon-deflected and fully deflected pixels.
 36. The method according toclaim 31, wherein said chessboard pattern is comprised of onlynon-deflected and partially deflected pixels.
 37. The method accordingto claim 35, wherein said fully deflected pixels corresponds to amaximum degree of extinction by means of diffraction.
 38. The methodaccording to claim 36, wherein said partially deflected pixelscorresponds to partial extinction by means of diffraction.
 39. Themethod according to claim 31, wherein said chessboard pattern iscomprised of only fully-deflected and partially deflected pixels. 40.The method according to claim 17, wherein said chessboard pattern iscomprised of pixels being in a first partially deflected state and asecond partially deflected state.
 41. The method according to claim 32,wherein said chessboard pattern is comprised of only non-deflected andfully deflected pixels.
 42. The method according to claim 32, whereinsaid chessboard pattern is comprised of only non-deflected and partiallydeflected pixels.
 43. The method according to claim 41, wherein saidfully deflected pixels corresponds to a maximum degree of extinction bymeans of diffraction.
 44. The method according to claim 42, wherein saidpartially deflected pixels corresponds to partial extinction by means ofdiffraction.
 45. The method according to claim 32, wherein saidchessboard pattern is comprised of only fully-deflected and partiallydeflected pixels.
 46. The method according to claim 32, wherein saidchessboard pattern is comprised of pixels being in a first partiallydeflected state and a second partially deflected state.
 47. The methodaccording to claim 31, further comprising the action of: identifying anSLM reference pixel in a detector pixel grid.